This invention relates to an electrophoretic medium with improved image stability, and to an electrophoretic display including this medium. More specifically, this invention relates to an electrophoretic medium and display which allow improved image stability without unacceptable increases in the switching time or the drive voltage of the display.
Electrophoretic displays have been the subject of intense research and development for a number of years. Such displays can have attributes of good brightness and contrast, wide viewing angles, state bistability, and low power consumption when compared with liquid crystal displays. The terms “bistable” and “bistability” are used herein in their conventional meaning in the art to refer to displays comprising display elements having first and second display states differing in at least one optical property, and such that after any given element has been driven, by means of an addressing pulse of finite duration, to assume either its first or second display state, after the addressing pulse has terminated, that state will persist for at least several times, for example at least four times, the minimum duration of the addressing pulse required to change the state of the display element. (In practice, some electrophoretic displays, including some of the displays of the present invention, are capable of achieving multiple gray states, and, as demonstrated below, are stable not only in their extreme black and white optical states, but also in their intermediate gray states. Although such displays should properly be described as “multi-stable” rather than “bistable”, the latter term may be used herein for convenience.) The optical property which is changed by application of an electric field is typically color perceptible to the human eye, but may be another optical property, such as optical transmission, reflectance, luminescence or, in the case of displays intended for machine reading, pseudo-color in the sense of a change in reflectance of electromagnetic wavelengths outside the visible range. Nevertheless, problems with the long-term image quality of these displays have prevented their widespread usage. For example, particles that make up electrophoretic displays tend to settle, resulting in inadequate service-life for these displays.
Numerous patents and applications assigned to or in the names of the Massachusetts Institute of Technology (MIT) and E Ink Corporation have recently been published describing encapsulated electrophoretic media. Such encapsulated media comprise numerous small capsules, each of which itself comprises an internal phase containing electrophoretically-mobile particles suspended in a liquid suspension medium, and a capsule wall surrounding the internal phase. Typically, the capsules are themselves held within a polymeric binder to form a coherent layer positioned between two electrodes. Encapsulated media of this type are described, for example, in U.S. Pat. Nos. 5,930,026; 5,961,804; 6,017,584; 6,067,185; 6,118,426; 6,120,588; 6,120,839; 6,124,851; 6,130,773; 6,130,774; 6,172,798; 6,177,921; 6,232,950; 6,241,921; 6,249,271; 6,252,564; 6,262,706; 6,262,833; 6,300,932; 6,312,304; 6,312,971; 6,323,989; and 6,327,072; U.S. patent application Publication No. 2001-0045934; and International Applications Publication Nos. WO 97/04398; WO 98/03896; WO 98/19208; WO 98/41898; WO 98/41899; WO 99/10767; WO 99/10768; WO 99/10769; WO 99/47970; WO 99/53371; WO 99/53373; WO 99/56171; WO 99/59101; WO 99/67678; WO 00/03349; WO 00/03291; WO 00/05704; WO 00/20921; WO 00/20922; WO 00/20923; WO 00/26761; WO 00/36465; WO 00/36560; WO 00/36666; WO 00/38000; WO 00/38001; WO 00/59625; WO 00/60410; WO 00/67110; WO 00/67327 WO 01/02899; WO 01/07691; WO 01/08241; WO 01/08242; WO 01/17029; WO 01/17040; WO 01/17041; WO 01/80287 and WO 02/07216. The entire disclosures of all these patents and published applications are herein incorporated by reference.
Many of the aforementioned patents and applications recognize that the walls surrounding the discrete microcapsules in an encapsulated electrophoretic medium could be replaced by a continuous phase, thus producing a so-called “polymer-dispersed electrophoretic display” in which the electrophoretic medium comprises a plurality of discrete droplets of an electrophoretic fluid and a continuous phase of a polymeric material, and that the discrete droplets of electrophoretic fluid within such a polymer-dispersed electrophoretic display may be regarded as capsules or microcapsules even though no discrete capsule membrane is associated with each individual droplet; see for example, WO 01/02899, at page 10, lines 6–19. See also copending application Ser. No. 09/683,903, filed Feb. 28, 2002, the entire disclosure of which is herein incorporated by reference.
An encapsulated, electrophoretic display typically does not suffer from the clustering and settling failure mode of traditional electrophoretic devices and provides further advantages, such as the ability to print or coat the display on a wide variety of flexible and rigid substrates. (Use of the word “printing” is intended to include all forms of printing and coating, including, but without limitation: pre-metered coatings such as patch die coating, slot or extrusion coating, slide or cascade coating, curtain coating; roll coating such as knife over roll coating, forward and reverse roll coating; gravure coating; dip coating; spray coating; meniscus coating; spin coating; brush coating; air knife coating; silk screen printing processes; electrostatic printing processes; thermal printing processes; ink jet printing processes; and other similar techniques.) Thus, the resulting display can be flexible. Further, because the display medium can be printed (using a variety of methods), the display itself can be made inexpensively.
Although as already mentioned, electrophoretic displays exhibit bistability, this bistability is not unlimited, and images on the display slowly fade with time, so that if an image is to be maintained for extended periods, the image must be refreshed periodically (and the intervals at which such refreshing is necessary is a convenient quantitative measure of image stability). Also, in many systems which lack image stability, it is necessary to apply so-called “blanking pulses” at regular intervals; such blanking pulses involve first driving all the pixels of the display to one optical state (for example, a white state), then driving all the pixels to the opposite optical state (for example, black), and then writing the desired image. Since such refreshing and blanking of the display inherently consumes energy, and the blanking pulses are distracting for a user who is trying to concentrate on an image, it is desirable to minimize such refreshing and blanking, that is to say, to increase the bistability of the display so that the intervals between refreshing and/or blanking of the image can be increased, and the power consumption of the display thus reduced. For example, one potential application of electrophoretic displays is in personal digital assistants (PDA's) where the inherent size and weight limitations of the device, and the need for the screen to be visible under a wide variety of lighting conditions, render low power consumption and reflective mode of electrophoretic displays very attractive. It is common for the user of a PDA to keep a single image, such as a list of telephone numbers, on the screen for an extended period, and in order to keep the energy consumption of the PDA and distractions to the user to an absolute minimum, it is desirable that this be achievable without the need for any refreshing or blanking of the display during this extended period.
It has been found, by the present inventor and his co-workers, that the main factor limiting image stability in electrophoretic displays of the types described in the aforementioned patents and applications is settling of the electrophoretic pigment particles under gravity. Since the rate of such settling is to a first approximation inversely proportional to the viscosity of the liquid phase in which the pigment particles are suspended, the stability of the image can be increased by increasing the viscosity of the liquid phase. Unfortunately, as is well known to those skilled in the technology of electrophoretic displays, the electrophoretic mobility of the pigment particles (the rate at which the particles move through the liquid phase under a given electric field) is also inversely proportional to the viscosity of the liquid phase, and thus the switching time of the display (the time required to change a given pixel of the display from one of its optical states to the other essentially the time necessary for the pigment particles to move through the thickness of the liquid medium) is directly proportional to the viscosity of the liquid medium. Accordingly, although it is well within the level of skill in the art to vary the viscosity of the liquid medium over a wide range, it has hitherto appeared that any chosen viscosity necessarily represents a compromise, in that increasing the viscosity will increase image stability at the cost of increased switching time. Furthermore, especially in the case of small displays such as PDA displays where it may be necessary to move through several “pages” or screens of information to find a desired datum, users will not tolerate switching times substantially in excess of about 1 second. (Although it is possible to counteract an increase in switching time due to increased viscosity of the suspending fluid by increasing the drive voltage applied to the display, this brings its own set of problems. Increasing the drive voltage necessarily increases the energy consumption of each driving pulse, and may increase the complexity and cost of the electronic circuitry required to control the driving pulses. Furthermore, in many battery-driven devices, it is not practicable to increase the driving voltage above certain limits.) Thus, it has hitherto appeared that the maximum image stability which can be achieved in a PDA or similar electrophoretic display is limited to a value which is substantially lower than is desirable for energy conservation purposes.
It has now been found that the addition of certain polymers to the suspending fluid used in electrophoretic displays provides an increase in image stability greater than can be accounted for by the increase in viscosity of the fluid caused by the addition of the polymer. Accordingly, the use of these polymers in the suspending fluid allows for substantial increases in image stability without excessive increase in the switching time of the display.